Overhead projectors are known in a variety of forms. Recently it has been sought by means of an overhead projector to project on a projection wall a picture of a liquid crystal indicating or display device laid on the writing plate of the overhead projector. This has been quite successful for a short time after the light source of the overhead projector is switched on, but then the picture becomes increasingly worse. It becomes blurred and some parts of the image formed by the liquid crystals disappear. If this experiment is repeated many times it is found that even in the initial stage the image always becomes worse. These disadvantages are attributable to the warming of the liquid crystal indicating device. The warming of the writing plate in the course of the projection leads likewise to the heat absorption in the liquid crystals and the transparent plates which enclose them so that first the arrangement of the liquid crystals is disturbed and scattered and later the liquid crystals lose their property of assuming particular arrangements under the influence of electric fields.
It has been sought to eliminate these disadvantages through an intensive cooling of the liquid crystal indicating device. With these attached ventilators which also blew air through a space between the liquid crystal indicating device and the writing plate a satisfactory reduction of the temperature of the liquid crystal device was not attained. Moreover, the vibration of the ventilators led to unsharpness of the picture.